1. Field of the Invention
The present invention relates to a projection liquid crystal display apparatus which incorporates a holographic optical element serving as color separation means. More particularly, the present invention relates to a projection liquid crystal display apparatus incorporating a holographic optical element having a function to focus reproduced images on each pixel of a liquid crystal display unit.
The present invention is also concerned with a process for producing the holographic optical element described above, as well as to a holographic optical element produced by the process.
2. Description of the Related Art
Liquid crystal display devices can be sorted into two types: namely, a so-called direct view type display apparatus in which an image formed on a liquid crystal display unit, e.g., a liquid crystal light valve or a liquid crystal display panel, is directly viewed, and a so-called projection type display apparatus, referred to as liquid crystal projector, in which an image is projected and displayed on a screen. In the operation of a projection liquid crystal display apparatus, white light emitted from a light source is separated into primary colors for color display, and the light of each color is guided to a pixel (liquid crystal cell) of a liquid crystal display unit which corresponds to the color of the light. The light is modulated in this pixel in accordance with the image to be reproduced, and the modulated light is projected on a screen so that a color image can be displayed on the screen at a greater scale. Such projection liquid crystal display apparatuses also can be sorted into two types: a single-sheet type which employs a single sheet of liquid crystal display unit having a tri-color separation means for separating white light into three colors of red (R), green (G) and blue (B), and a triplet sheet type which employs three sheets of monochromatic liquid crystal display units for the light paths of red, green and blue colors.
In most cases, liquid crystal display units developed for use in direct view type apparatuses are conveniently used as the single sheets of liquid crystal display units in projection liquid crystal display apparatus of single-sheet type. In general, such a liquid crystal display units primarily intended for use in direct view type display apparatuses has color filters adhered to realize the required color display.
Conventional liquid crystal display apparatuses employing color filters bonded to the liquid crystal display units have suffered from the following problems. In this type of display apparatus, a red color filter is disposed on a pixel which is intended to display red color. This red color filter inconveniently absorbs or reflects light of other colors, i.e., green and blue. This is also the cases of other color filters, i.e., green and blue color filters. Thus, the known display apparatus of the type described could use only 1/3+L the light rays incident to the liquid crystal display unit. Thus, the efficiency of use of the light is lower than that in projection liquid crystal display apparatus of triplet sheet type using the same light source. For the same reason, the brightness of the display on the screen is reduced to 1/3. It would be possible to enhance the brightness by increasing the power of the light source, but such a countermeasure undesirably incurs a temperature rise due to absorption of light. This means that the liquid crystal display unit of the type having color filters encounters a practical limit in the brightening of the display.
In order to obviate the disadvantage of the color filters, a method has been proposed in, for example, Japanese Patent Laid-Open No. 4-60538, in which dichroic mirrors and microlenses are combinedly used in place of the color filters so as to realize color display. More specifically, in which method, white light emitted from a light source is separated into three color components by means of dichroic mirrors which are slanted differently for three primary colors, and the separated color light rays are made to be incident to the corresponding pixels of the liquid crystal display unit through the microlenses. This color displaying method, however, involves problems such as increase in the size of the optical system, reduction in the color purity due to mixing of color, i.e., noise, and so forth. In addition, the whole display apparatus becomes costly due to the use of expensive microlenses.
The method relying upon the use of dichroic mirror and microlenses further suffers from the following disadvantage. Namely, the color separation into three primary colors of red, green and blue, performed by dichroic mirrors, is possible only in a common plane. Consequently, the pixels on the liquid crystal display unit can be arranged only linearly, e.g., along a horizontal line and along a line perpendicular thereto. The use of a liquid crystal display unit having such linear pixel arrangement poses a restriction in the construction of the optical system. In addition, the microlenses have to be shaped linearly in accordance with the pixel arrangement on the liquid crystal display unit, with the result that a difficulty is encountered in the production of the microlenses. On the other hand, the liquid crystal display unit employs a so-called delta or triangle pixel arrangement in which pixels of red, green and blue colors are arranged to form a triangular pixel module, the size of each microlens corresponding to each triangular pixel module has to be increased beyond the size actually required, with the result that a wasteful portion which does not contribute to the condensation of light is inevitably involved in each microlens.
Still further problem encountered by this type of projection liquid crystal display apparatus is that, when a light source which emits white light is used, color purity is undesirably impaired due to presence of complementary color components.